[viking499]

What is the differenc between these two? Just the primers?

[arcticap]

Magnum doesn't refer to any difference in primers although there are different 209 primers to choose from.
The Magnum designation is a marketing label that usually refers to the barrel being rated for use of up to 150 grains of powder instead of 100 grains [by volume].
Some guns can handle 150 grains of loose ffg powder or pellets interchangably, while others only recommend 150 grains of pellets and less if using loose powder.
Loose 777 powder loads always need to be reduced by 15% volume, and fffg loads need to be reduced by about 10% to be equivalent to ffg powder loads.

Here's a previous thread discussing the magnum marketing designation:

http://www.thehighroad.org/showthread.php?t=346024&highlight=magnum